GB2213762A - Manufacture of ceramic shell moulds - Google Patents

Abstract

A ceramic shell is made by applying layers of water-based composition comprising refactory material and a binder, and a stucco to a pattern made of a cellular plastics material and drying each layer under controlled conditions, and then removing the pattern.

Description

MANUFACTURE OF CERAMIC SHELL MOULDS The invention relates to the manufacture of ceramic shell moulds, and the moulds so made. A ceramic shell mould is useful in the casting of molten metals.

A ceramic shell mould is made by applying a succession of coatings of a slurry composition and stucco to a pattern1 hardening the binder in each coating in turn and then removing the pattern. Typically there are one or two primary coatings followed by a number of secondary coatings.

The pattern may be a wax or a synthetic resin which may be a solid or in expanded form as in the case of polystyrene foam.

The slurry composition applied to the pattern comprises a refractory material such as a zircon flour, aluminosilicate flour, silica, alumina and the like, together with a binder therefor. The binder is typically a silicate, and two classes of silicate are used. In so called water-based systems, the binder is a colloidal silica sol, and in so called alcohol-based systems the binder is ethyl silicate.

While the refractory slurry on the pattern is still wet a refractory stucco is applied, such as fused alumina, zircon sand, alumino-silicate, silica and the like. In an alcoholbased system the binder is hardened or set either by air drying (i.e. solvent evaporation) or more rapidly by exposure to ammonia. In a water-based system the binder is hardened or set by air drying (i.e. solvent evaporation).

Both systems have their advantages and disadvantages.

Water-based binder compositions are relatively cheap but a prolonged drying period is necessary. This can range from about 8 hours for a primary coating to about 16 hours in the case of a secondary coating. Where a ceramic coating is being applied to a wax pattern it is not possible to accelerate the drying of a coating derived from a waterbased binder composition by heating because the wax pattern is temperature sensitive, and any heating will distort the pattern. For larger castings, where the ceramic mould requires considerable strength for handleability, it may be necessary to invest the pattern with as many as 16 coatings, and the total mould making time can be as long as two weeks.

In some foundries all the coatings are made using a water based binder composition. This is the technique commonly practised in the U.S-.A. In other countries, where the fire hazard regulations are not so severe, it is usual to use water-based binder in the primary coating or coatings and alcohol-based binder in subsequent coatings.

Alcohol-based binder systems require less drying time but they can only be used where local regulations permit and even then expensive precautions must be taken because such systems are classified as inflammable liquids (flashpoint 110C). The virtue of the use of an alcohol-based system is that it can be rapidly cured by exposure to ammonia so the foundry needs less working and storage space, which is important especially in the case of larger patterns.

It has been proposed to make a ceramic shell mould using a pattern comprising a cellular plastics material such as expanded polystyrene. The use of such a material offers considerable advantages, especially when making larger castings which entails the use of larger patterns, as a wax pattern will typically be twenty times heavier than a polystyrene pattern. For these large components the shell formed will normally include a number of alcohol-based coatings in order to have the advantages of faster drying coupled with low storage space and inventory.

It is one object of the invention to provide a method of making ceramic shell moulds which uses a water-based binder composition but which does not involve a prolonged drying step for each coating. The invention is based on the realisation that by making the pattern of a cellular plastics material and using such a water-based binder it is possible to dry the coating formed thereon quickly, provided the conditions of the drying step are controlled.

According to a first aspect of this invention there is provided a method of making a ceramic shell mould for use in casting a metal article, the method comprising forming a combustible pattern of cellular plastics material corresponding in shape and size to the article to be cast, applying successive hardenable coatings of refractory slurry and stucco, drying and removing the pattern, characterised by the steps of (i) applying to the pattern successive coatings of a slurry comprising a refractory flour and a water-based binder, and a stucco; (ii) drying each coating by subjecting it for a period to a coating drying temperature below that at which the cellular plastics material of the pattern undergoes a significant decrease in volume.

The drying of each coating is preferably carried out according to the invention by placing the pattern carrying the coating to be dried in an oven, and most preferably circulating heated air or other drying gas about the pattern.

The temperature at which the cellular plastics material will undergo an adverse dimensional change will vary from material to material. In the case of a material such as expanded polystyrene, the material undergoes a linear expansion reaching a maximum at about 60-700C, followed by a shrinkage above about 1000C (Fig 1). The maximum linear expansion of expanded polystyrene is about 0.15%, and this low level is believed to be due to the low inherent expansion of the material and its low thermal conductivity.

The linear expansion which does occur with expanded polystyrene is normally insufficient to deform or crack the shell and while the drying temperature may be above 60-700C our evaluations have shown that the most satisfactory coatings are obtained when drying temperatures below 600C are used. When the rattern is heated above about 1000C, the material undergoes a significant decrease in volume ("shrinkage") and that has adverse effects, e.g. weakness in the coating.

In a much preferred aspect of the invention therefore there is provided a method as defined, including the steps of a) applying to the pattern a coating of refractory slurry comprising a refractory flour and a water-based binder, b) applying a refractory stucco so that it adheres to the wet slurry on the pattern, c) subjecting the pattern coated by steps (a) and (b) to a drying step comprising circulating air or other gas at about 400C to about 600C, most preferably about 500C, for a period of from about 15 to about 60 minutes, most preferably about 30 minutes, and a) removing the pattern from the shell of dried coatings so as to leave a ceramic shell mould.

For some pattern configurations (e.g. large flat areas) it is preferable to dry the primary coating or coatings at a lower temperature between about 20 and 300C preferably about 220C to obtain a satisfactory result. This lower temperature is also used if any areas of the pattern have been upgraded by the application of a wax coating.

Most preferably the circulating drying gas is at a minimum velocity of about two metres/second, otherwise the drying time is excessive.

This invention enables the drying period in the preparation of a ceramic shell mould to be reduced to a matter of hours instead of days using a water-based binder system when the emphasis of the industry is to use alcohol-based binder systems to reduce the drying time. This is achieved according to the invention by using a cellular plastics material for the pattern and controlling as defined the conditions of the drying.

The invention also offers the potential of manufacturing more dimensionally accurate castings using ceramic shell moulds manufactured from expanded polystyrene patterns. It is known that expanded polystyrene patterns undergo a dimensional contraction and do not become stable until about 40 days after moulding Using the technique described here it is possible to apply the ceramic shell coating almost immediately after moulding the expanded polystyrene pattern and hence before any significant dimensional changes have occurred. In the alcohol-based system the expanded polystyrene patterns must be thoroughly dried after moulding therefore a greater dimensional change will have taken place before the alcohol-based refractory coating is applied.

It is a preferred feature of the invention that before the pattern is removed from the shell the coated pattern is subjected to a preliminary extra heating step. This intermediate temperature treatment ensures that the coating is sufficiently dry for firing to take place as well as stabilising the pattern so that it does not crack the shell during firing. This extra step is carried out between 900C and 2000C, preferably about 900C, for a period of between 30 to 60 minutes, preferably 60 minutes.

Most preferably the pattern is removed from the coating by the technique of subjecting the coated pattern to the application of rapid heat at a temperature of from about 8000C to 11000C for a period of about 5 to about 15 minutes.

This technique is described and claimed in our European Patent Application 84.300309.6, publication A-0115402 (My Ref: 3691), all the disclosure of which is intended to be incorporated herein merely by this reference.

The coating composition may comprise any known composition of a refractory flour and a suitable water-based binder system therefor, typically an aqueous colloidal silica sol.

Such colloidal silica sols are available commercially in a range of Si02 contents and particle size. The invention has been shown to work well using a colloidal silica sol having a small particle size and a nominal SiO2 content of about 30g by weight. The invention however is not limited to the use of this particular binder only. Where necessary the mould permeability can be increased by dilution of the binder with distilled water and good results have been obtained with a diluted binder containing effectively 208 silica.

The invention includes a ceramic shell mould when made by the method. It must be emphasised that both the refractory flour and the stucco must be present in the coating to be dried if a ceramic shell mould is to be formed otherwise the pattern cannot be removed to form a self supporting shell mould. This mould may be used in any suitable metal casting process, for example one using supported or unsupported shells and with or without the application of vacuum, especially to form castings of relatively large size or section thickness. It is a much preferred feature of this invention that the casting be carried out according to the techniques of our earlier European publication A-0115402 to form castings of the metals and dimensions indicated in that document.

The invention is illustrated by way of example with reference to the accompanying Figures rn which Figure 2 shows the weight loss of a primary coating when in air circulated at 2 metres per second at a temperature of 500C, and Figure 3 shows the same data in the case of a secondary coating when in air circulated at 2 metres per second at 500C.

The primary coating was formed of a composition comprising a slurry of zircon flour and an aqueous colloidal silica sol binder which was applied in known manner to a pattern of expanded polystyrene, and a refractory stucco of fused alumina (0.25 - 0.5 mm) which was applied in known manner to the slurry coated polystyrene pattern. The coated pattern was then placed in a heated oven at 50 C with the air circulated at 2 metres per second and the weight determined over time. It was observed that after about 5 minutes the weight became substantially constant showing the water had been driven off and a binder had bonded the refractory particles. The pattern was then invested with a secondary coating of slurry and stucco; the slurry composition comprising a molochite flour and a diluted aqueous colloidal sol binder (equivalent silica content 20t), and the stucco being a 16-30 grade of molochite. The coated pattern was dried at 500C in air circulated at 2 metres per second (see Figure 3). results show that by virtue of the invention it is possible to use a water-based binder composition which can be dried quickly.

Claims (14)

1. A method of making a ceramic shell mould for use in casting a metal article, the method comprising forming a combustible pattern of cellular plastics material corresponding in shape and size to the article to be cast, applying successive hardenable coatings of refractory slurry and stucco, drying and removing the pattern, characterised by the steps of (i) applying to the pattern successive coatings of a slurry comprising a refractory flour and a water-based binder, and a stucco; (ii) drying each coating by subjecting it for a period to a coating drying temperature below that at which the cellular plastics material of the pattern undergoes a change in volume.

2. A method according to Claim 1 characterised in that the drying of each coating is carried out by placing the pattern carrying the coating to be dried in an oven and circulating heated air or other drying gas about the pattern.

3. A method according to Claim 2 characterised in that the circulating drying gas is circulated at a minimum velocity of about two metres/second.

4. A method according to Claim 1, 2 or 3 characterised in that the cellular plastics material of the pattern is expanded polystyrene and drying temperatures below 600C are used.

5. A method according to Claim 4 characterised by subjecting the coated pattern to a drying temperature of about 30 C to about 600C, for a period of from about 15 to about 60 minutes.

6. A method according to Claim 5 characterised in that the drying is carried at about 500C for about 30 minutes.

7. A method according to any preceding Claim characterised by drying the primary coating or coatings at a temperature between about 20 and 300C.

8. A method according to any preceding Claim characterised in that the expanded polystyrene pattern is subjected to the ceramic shell coating immediately after moulding and before the expanded polystyrene pattern has undergone any significant dimensional change.

9. A method according to any preceding Claim characterised in that the coating composition comprises a refractory flour and a water-based binder which comprises a colloidal silica sol having a small particle size and a nominal SiO2 content of about 30% by weight.

10. A method according to any preceding Claim characterised in that the silicate binder of the slurry applied to the pattern when already coated with a first coating has an effective silica content of about 20%.

11. A method according to any preceding Claim characterised in that before the coated pattern is removed from the shell the coated pattern is subjected to a preliminary extra heating step at between 900C and 2000C, for a period of between 30 to 60 minutes.

12. A method according to Claim 11 characterised in that the extra heating step is carried out at about 900C for about one hour.

13. A method according to any preceding Claim characterised in that the pattern is removed from the coating by subjecting the coated pattern to the application of rapid heat at a temperature of from about 8000C to 11000C for a period of about 5 to about 15 minutes.

14. A ceramic shell mould when made by a method according to any preceding Claim.